Conversation: Sony’s Andrew Bubala on Digital Noise-Cancelling

Posted by: Chris Martens at 5:05 pm, December 12th, 2008

Earlier this week I had an opportunity to talk with Andrew Bubala, director of marketing for audio accessories, Sony Electronics, on the subject of digital noise cancelling technology for headphones. Actually, the call was partly to follow up with Andrew on my review of Sony’s MDR-NC500D, which appears in the current issue of Playback magazine, and partly an opportunity for me to get a deeper, “insider’s” view on the thinking behind the MDR-NC500D. I thought I’d share some of things I learned.

Sony’s MDR-NC500D is, as some of you already know, a so-called “feedback-type” noise cancelling headphone, meaning that it’s noise-sensing microphone is placed inside the headphone earcup. In this way, the mic “listens to” the same noises, and from the same vantage point, that you do. The concept, of course, is that the mic’s output signal can therefore be used to create an offsetting noise-cancellation signal that will (to a significant degree) cancel noise. This approach does mean that Sony, and all other feedback-type noise-cancellers, must provide some mechanism that allows the headphone’s circuitry to discriminate between noise and music (which the mic also picks up). Otherwise, the system would attempt to cancel out both noise and your music.

Andrew pointed out that Sony’s MDR-NC500D is not unique in being a feedback-type design; Bose’s ubiquitous QuietComfort2 (QC2) headphone is also a feedback-type design as is Sony’s earlier MDR-NC60. But four things are different about the MDR-NC500D:

1.    First, the Sony—unlike the Bose QC2, which uses analog noise cancellation circuitry—converts inbound music and noise signals into digital format and only then applies digital noise filters and EQ-shaping to simultaneously cut down noise and improve sound quality.  Andrew stressed that it is far easier to analyze noise and to compare noise vs. music signals in the digital domain.

2.    Second, the Sony—unlike any other headphone we know of—offers three different digital noise filters, one optimized for airplanes, another for trains/busses, and a third for office/study environments. Andrew explained that while all noise-cancellers are, to some degree, capable of adapting to the noise environment around them, the noise spectrums of planes, trains, and studies are sufficiently different that it pays to have different noise filter curves for each.

3.    Third, the Sony provides an artificial intelligence (AI) circuit that monitors noise when you first fire-up the headphones, and then chooses the appropriate filter curve for your environment. To activate the AI circuit, you just push a button on the outside of the earcup and wait a bit. From the inside, you hear the music stop for a few seconds (so that all you hear is whatever noise penetrates the earcups), followed by a chime-like chirp, at which point you’ll simultaneously hear the noise floor drop dramatically and the music resumes, sounding clearer than ever. Andrew indicated that part of the beauty of the AI circuit is that it can analyze noise faster and better than most humans can and that it always “chooses the optimal filter curve for the circumstance.” (Note: You can manually override the AI circuit’s decision if you want to try other curves, but my experience was that it does indeed make the best judgment call).

4.    Fourth, the Sony can apply digital EQ to help “clean up” or “enhance” the inbound music signal. All processing, again, happens in the digital domain using a beefy DSP engine, so that digital audio data is only converted back into analog signals at the last moment, just before being used to drive the headphone’s transducers. Since noise is essentially subtracted from the audio signal in a very precise way, Andrew said, the MDR-NC500D is able to preserve a much higher degree of musical subtlety—especially through the critical midrange—than most other noise cancelling headphones based on analog noise cancellation circuits.

Toward the end of our talk, I got a gratifying bit of confirmation that the observations I offered in my review of the MDR-NC500D were on target. I had said the headphones generally had neutral tonal balance but that they seemed to introduce a bit of bass boost, way down low. “Yes, we were frankly surprised that you picked up on that,” said Andrew. As it turns out, the U.S. model MDR-NC500D has a somewhat different tonal balance curve than the Japanese-spec model. The Japanese model offers ruler-flat frequency response, while the U.S, model introduces—you guessed it—a very slightly enriched, thicker bottom end.